Doctor Who (Tom Baker) receives a distress call from Saturn's moon
Titan, only to find all the crew overtaken by some aliens, and worse yet,
he is taken over by the head alien, the Nucleus (John Scott Martin), who
starts growing inside his brain. Fortunately, with the help of his
companion Leela (Louise Jameson), the Doctor can make it to the next
hospital asteroid, where Doctor Marius (Frederick Jaeger) makes miniature
clones of him and Leela and injects them into his brain to fight the
Nucleus mano a mano. Thing is, ultimately the hospital is overrun by
humans possessed by aliens, and Leela and Doc Marius' robot dog K9 (voiced
by John Leeson) are no match for the invading forces - to an extent where
even Marius is taken over.

But Doctor Who and Leela's clones manage to
not only drive the Nucleus out of the Doctor's system, they also find the
antidote in the Doctor's brain - and ultimately cure Marius and his
assistants, while the Nucleus (now grown to man-size) makes it off to
Titan with a few of those still possessed - but the Doctor blows the Titan
space station right out of the sky ...

And as a thank you, the Doctor
and Leela are awarded with K9 - who will become a regular on the series
for the next few years.

On a quality level, this episode falls
between many stools: On one hand, it features impressive miniature
effects, maybe the best oft he show, on the other hand, the Nucleus is one
of the silliest looking monsters ever, on one hand, the designs of the
Doctor's brain's inside are very impressive, on the other hand, the scenes
inside his brain are very sloppily executed. And then there are several
special effects that just don't work, period, be it a piece of
concrete cut out of a wall (if you watch it, you'll know what I mean) or
simply K9 shooting laserbeams from his nostrils.

On a story level, this
episode is trashy fun, but considering all the concepts thrown in, it
doesn't seem all that thought through and is certainly not one of the
better episodes of the series.